Disc brake systems are well known, having been used extensively in the automotive industry. These systems are produced in a wide variety of sizes, types, and configurations, but the basic function of disc brakes is to convert kinetic energy, the energy of motion, into thermal energy, or heat. As designed, the friction between disc brake pads and brake rotors during braking generates large quantities of heat as brake systems convert kinetic energy into heat to slow or stop moving vehicles.
Nearly all disc brake systems are comprised of certain basic components. Among them is a caliper that surrounds at least a portion of a brake rotor. Mounted in the caliper are brake pads which are designed to press against the rotor causing the disc brake to slow the vehicle using the brake system. The pressing action is accomplished by a force producing element, usually a hydraulic system, which causes one or more brake pistons to press against at least one brake pad forcing the pad against the rotor and causing frictional forces to change the kinetic energy of the vehicle into heat.
Disc brakes can generate intense heat creating thermal gradients of several hundred degrees between disc brake system parts and their surroundings. This intense heat can adversely affect mechanical and hydraulic systems. As part of the ongoing effort to improve brake efficiency and increase brake system longevity, many methods and devices have been developed to help dissipate the intense heat generated during braking. In addition, various approaches have been developed to insulate and prevent boiling of brake fluid from the heat generated.
The problem of heat generation during braking is made much worse than it otherwise would be by irregularities in brake rotors. As a common problem with disc brakes, irregularities cause generation of excessive heat at brake pad to rotor interfaces. The problem arises as the rotors are forced between opposing brake pads during braking. The irregularities cause uneven frictional drag resulting in excessive braking temperatures and in turn resulting in significant adverse effects on braking efficiency and brake system longevity.
The problem is endemic to disc brakes because it is unavoidable that rotors distort during use. All rotors develop irregularities on their friction faces. These irregularities are sometimes referred to as high spots, a term that can apply to any type of rotor irregularity, warping, or rotor disc thickness variation.
All types of rotor distortion have the same effect on braking dynamics. If not already present, high spots on rotor faces form and begin to grow in magnitude as soon as rotors are put into service. Although they tend to develop gradually during normal brake use, they begin to affect braking dynamics as soon as they begin to form and long before their presence becomes noticeable to drivers as a pulsating pedal.
When brakes are applied, and brake pads are pressed against rotating rotor faces, high spots on the rotor faces are forced to squeeze between the pads as the rotors rotate. The brake pads resist any lateral or backward movement of the pads perpendicular to and away from the face of the rotor. As the high spots on the rotor faces pass between pads applied during braking, severe and excessive frictional drag is produced. This generates much more heat than would be the case if there were no high spots on the rotors.
The fact that brake systems are designed to generate tremendous quantities of heat has obscured recognition of the role that rotor high spots play in the generation of excessive friction and heat.
Excessive heat generated by high-spot friction often is the underlying cause of reduced braking efficiency, accelerated brake wear, and other problems. These proxies comprise:
Extended stopping distance. During a typical braking cycle from brake application to brake release, temperatures at pad to rotor interfaces routinely climb above the efficiency threshold where braking efficiency is sharply reduced because the temperatures at the brake pad/rotor interface are above the design temperature for efficient braking. This lowers friction coefficients, reduces brake system efficiency, and extends normal stopping distances.
Brake fade. During heavy, frequent, or prolonged braking cycles, pad to rotor interface temperatures can reach or exceed 538 degrees Centigrade (C.) or 1,000 degrees Fahrenheit (F.), which results in the dangerous condition known as brake fade or loss of friction.
Accelerated pad wear. As braking temperatures increase, the rate of wear of brake friction material on brake pads also increases.
Rotor degradation. Rotors, when used at temperatures above the efficiency threshold, exhibit a greater tendency for scoring, glazing, cracking, and blue-spotting caused by localized hot spots.
Shortened caliper life. The service life of calipers is also shortened by excessive heat. Excessive heat is the primary cause of caliper boot and seal failure, and is the major factor in caliper seizing caused by heat-compromised lubricants.
Still other problems can be caused by severe frictional drag at high spots, which can be related more directly to frictional drag itself than to the excessive heat it generates. For example, during even moderate brake applications, because of the severe frictional drag which occurs at rotor high spots, there is an increased danger of one or more wheels locking up. When a wheel locks up, it nearly always occurs where a rotor high spot passes under a brake pad. High-spot friction can become greater than tire-to-road friction, especially when road surfaces are slippery or tires are worn. This danger becomes acute by the time high spots have grown to such proportions that they are noticeable to the driver as a pulsating pedal.
Another problem is sway. During any braking cycle, the frictional drag at high spots on rotating rotor faces creates an uneven transfer of braking torque through the tires to the roadway, which reduces vehicle stability and increases the tendency of the vehicle to sway, or exhibit uneven side-to-side stability. This effect is especially evident on unevenly-loaded or high-bodied vehicles.
The uneven transfer of braking torque through the tires to the roadway also stresses tire assemblies, which causes external tire problems such as cupping and contributes significantly to internal tire problems such as cord separation.
One method for solving the problems described above could involve increasing the size of disc brake systems by using larger contact area and thicker rotors along with larger area brake pads, which would require larger calipers and redesigned master cylinders and other brake system parts. This method would help eliminate the formation of high spots on rotor faces by improving the ratio of friction material area to vehicle weight and by increasing the heat-sink capabilities of brake assemblies, resulting in lower brake operating temperatures. However, this method is usually not used because it would add undesired weight to vehicles and would be prohibitively expensive to implement.
Another method could involve providing a cushion, such as a hydraulic accumulator, inside the brake hydraulic system, which, if designed and calibrated correctly, could reduce high-spot friction and provide more efficient braking. Several devices of this type have been proposed, but these devices are usually designed to absorb hydraulic shock, and do not allow for the role that high spots play in the generation of excessive heat during braking. However, even if such a device were designed and calibrated specifically to provide optimum high-spot friction compensation, it would likely find reluctant acceptance because of cost and liability issues.
A simple and cost-effective method for reducing the excessive and uneven heat caused by high spot friction is needed.